The purpose of the proposed studies is to: 1) compare isogravimetric capillary hydrostatic pressures in skin and skeletal muscle of the horse and dog; 2) determine the effect of histamine on capillary pressure in the same tissue; 3) determine the effect of local histamine on microvascular permeability of skin and skeletal muscle in both horse and dog, and 4) determine if catecholamines, time and/or histamine blood concentrations are factors in the permeability effect of histamine on the microvasculature. Studies will be conducted on the horse gracilis muscle and digit and the dog gracilis muscle and forelimb. Both in vivo and in vitro studies will be conducted. All studies will utilize constant pressure perfusion. Uptake and washout of radioactive 131I albumin and 86Rb Cl in maximally dilated vascular beds will be used to determine permeability.